High-carbon hot-rolled steel sheet and method for manufacturing the same

ABSTRACT

A high-carbon hot-rolled steel sheet and a method for manufacturing the steel sheet are provided. The high-carbon hot-rolled steel sheet has a particular chemical composition. The microstructure of the steel sheet includes ferrite, cementite, and pearlite that accounts for 6.5% or less of the entire microstructure by area fraction. The proportion of the number of cementite grains having an equivalent circle diameter of 0.1 μm or less to the total number of cementite grains is 20% or less, the average cementite grain size is 2.5 μm or less, and the cementite accounts for 3.5% or more and 10.0% or less of the entire microstructure by area fraction. The average concentration of solute B in a region extending from a surface layer to a depth of 100 μm is 10 mass ppm or more. The average concentration of N present as AlN in the region is 70 mass ppm or less.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2020/000783, filedJan. 14, 2020, which claims priority to Japanese Patent Application No.2019-013957, filed Jan. 30, 2019, the disclosures of these applicationsbeing incorporated herein by reference in their entireties for allpurposes.

FIELD OF THE INVENTION

The present invention relates to a high-carbon hot-rolled steel sheethaving high cold workability and high hardenability (immersion-quenchhardenability and carburizing hardenability) and a method formanufacturing the high-carbon hot-rolled steel sheet.

BACKGROUND OF THE INVENTION

Currently, automotive parts such as transmissions and seat recliners areoften produced by processing hot-rolled steel sheets (high-carbonhot-rolled steel sheets) which are carbon steels for machine structuraluse specified in JIS G4051 and alloy steels for machine structural useinto desired shapes through cold working and then subjecting theresultants to quenching treatment to ensure the desired hardness. Thus,the hot-rolled steel sheets used as materials are required to have highcold workability and high hardenability, and various steel sheets havepreviously been proposed.

For example, Patent Literature 1 discloses a high-carbon steel sheet forfine blanking. The steel sheet has a chemical composition containing, bywt %, C: 0.15% to 0.9%, Si: 0.4% or less, Mn: 0.3% to 1.0%, P: 0.03% orless, T. Al: 0.10% or less, and one or more of Cr: 1.2% or less, Mo:0.3% or less, Cu: 0.3% or less, and Ni: 2.0% or less, or Ti: 0.01% to0.05%, B: 0.0005% to 0.005%, and N: 0.01% or less and has amicrostructure in which carbide grains having a spheroidization ratio of80% or more and an average grain size of 0.4 to 1.0 μm are dispersed inferrite.

Patent Literature 2 discloses a high-carbon steel sheet with improvedworkability. The steel sheet has a chemical composition containing, bymass %, C: 0.2% or more, Ti: 0.01% to 0.05%, and B: 0.0003% to 0.005%and has an average carbide grain size of 1.0 μm or less, with theproportion of carbide grains having a grain size of 0.3 μm or less being20% or less.

Patent Literature 3 discloses a B-alloyed steel that contains, by mass%, C: 0.20% or more and 0.45% or less, Si: 0.05% or more and 0.8% orless, Mn: 0.5% or more and 2.0% or less, P: 0.001% or more and 0.04% orless, S: 0.0001% or more and 0.006% or less, Al: 0.005% or more and 0.1%or less, Ti: 0.005% or more and 0.2% or less, B: 0.001% or more and0.01% or less, and N: 0.0001% or more and 0.01% or less, and,furthermore, one or more components selected from Cr: 0.05% or more and0.35% or less, Ni: 0.01% or more and 1.0% or less, Cu: 0.05% or more and0.5% or less, Mo: 0.01% or more and 1.0% or less, Nb: 0.01% or more and0.5% or less, V: 0.01% or more and 0.5% or less, Ta: 0.01% or more and0.5% or less, W: 0.01% or more and 0.5% or less, Sn: 0.003% or more and0.03% or less, Sb: 0.003% or more and 0.03% or less, and As: 0.003% ormore and 0.03% or less.

Patent Literature 4 discloses a steel for machine structural use withimproved cold workability and improved low decarbonization properties.The steel has a chemical composition containing, by mass %, C: 0.10% to1.2%, Si: 0.01% to 2.5%, Mn: 0.1% to 1.5%, P: 0.04% or less, S: 0.0005%to 0.05%, Al: 0.2% or less, Te: 0.0005% to 0.05%, and N: 0.0005% to0.03%, furthermore, Sb: 0.001% to 0.05%, and, in addition, one or moreof Cr: 0.2% to 2.0%, Mo: 0.1% to 1.0%, Ni: 0.3% to 1.5%, Cu: 1.0% orless, and B: 0.005% or less, and has a microstructure composed mainly offerrite and pearlite, with the ferrite grain size number being 11 ormore.

Patent Literature 5 discloses a high-carbon hot-rolled steel sheet withimproved hardenability and improved workability. The steel sheetcontains, by mass %, C: 0.20% to 0.40%, Si: 0.10% or less, Mn: 0.50% orless, P: 0.03% or less, S: 0.010% or less, sol. Al: 0.10% or less, N:0.005% or less, and B: 0.0005% to 0.0050%, further contains one or moreof Sb, Sn, Bi, Ge, Te, and Se in an amount of 0.002% to 0.03% in total,has a microstructure composed of ferrite and cementite, with the densityof cementite in ferrite grains being 0.10/μm² or less, and has ahardness of 75 or less in terms of HRB and a total elongation of 38% ormore.

Patent Literature 6 discloses a high-carbon hot-rolled steel sheet withimproved hardenability and improved workability. The steel sheetcontains, by mass %, C: 0.20% to 0.48%, Si: 0.10% or less, Mn: 0.50% orless, P: 0.03% or less, S: 0.010% or less, sol. Al: 0.10% or less, N:0.005% or less, and B: 0.0005% to 0.0050%, further contains one or moreof Sb, Sn, Bi, Ge, Te, and Se in an amount of 0.002% to 0.03% in total,has a microstructure composed of ferrite and cementite, with the densityof cementite in ferrite grains being 0.10/μm² or less, and has ahardness of 65 or less in terms of HRB and a total elongation of 40% ormore.

Patent Literature 7 discloses a high-carbon hot-rolled steel sheet thatcontains, by mass %, C: 0.20% to 0.40%, Si: 0.10% or less, Mn: 0.50% orless, P: 0.03% or less, S: 0.010% or less, sol. Al: 0.10% or less, N:0.005% or less, and B: 0.0005% to 0.0050%, further contains one or moreof Sb, Sn, Bi, Ge, Te, and Se in an amount of 0.002% to 0.03% in total,with the proportion of the amount of solute B to the B content being 70%or more, has a microstructure composed of ferrite and cementite, withthe density of cementite in ferrite grains being 0.08/μm² or less, andhas a hardness of 73 or less in terms of HRB and a total elongation of39% or more.

Patent Literature 8 discloses a high-carbon hot-rolled steel sheet thathas a composition containing, by mass %, C: 0.15% to 0.37%, Si: 1% orless, Mn: 2.5% or less, P: 0.1% or less, S: 0.03% or less, sol. Al:0.10% or less, N: 0.0005% to 0.0050%, B: 0.0010% to 0.0050%, and atleast one of Sb and Sn in an amount of 0.003% to 0.10% in total andsatisfying the relationship 0.50≤(14[B])/(10.8[N]), with the balancebeing Fe and unavoidable impurities, has a microstructure composed of aferrite phase and cementite, with the average grain size of the ferritephase being 10 μm or less, the spheroidization ratio of cementite being90% or more, and has a total elongation of 37% or more.

PATENT LITERATURE

-   PTL 1: Japanese Unexamined Patent Application Publication No.    2009-299189-   PTL 2: Japanese Unexamined Patent Application Publication No.    2005-344194-   PTL 3: Japanese Patent No. 4012475-   PTL 4: Japanese Patent No. 4782243-   PTL 5: Japanese Unexamined Patent Application Publication No.    2015-017283-   PTL 6: Japanese Unexamined Patent Application Publication No.    2015-017284-   PTL 7: International Publication No. 2015/146173-   PTL 8: Japanese Patent No. 5458649

SUMMARY OF THE INVENTION

The technique described in Patent Literature 1 relates to fine blankingproperties, and the influence of the dispersion morphology of carbide onthe fine blanking properties and hardenability is described.Specifically, Patent Literature 1 states that a steel sheet withimproved fine blanking properties and improved hardenability can beobtained by controlling the average carbide grain size to 0.4 to 1.0 μmand the spheroidization ratio to 80% or more. However, Patent Literature1 does not discuss cold workability and does not describe carburizinghardenability.

The technique described in Patent Literature 2 focuses on the fact thatnot only the average carbide grain size but fine carbide grains having asize of 0.3 μm or less have an influence on workability, and controlsthe average carbide grain size to 1.0 μm or less and also controls theproportion of carbide grains having a size of 0.3 μm or less to 20% orless. Patent Literature 2 states that this control provides a steelsheet with improved workability and discloses a steel sheet furthercontaining Ti and B and having high hardenability. However, PatentLiterature 2 does not describe, for example, solute B which influenceshardenability and does not state that the quenching hardness isdetermined in what area of the steel sheet.

According to the technique described in Patent Literature 3, a steelwith improved cold workability and improved decarbonization resistancecan be obtained by adjusting the chemical composition. However, PatentLiterature 3 does not describe immersion-quench hardenability orcarburizing hardenability.

According to the technique described in Patent Literature 4, theincorporation of B and one or more components selected from Cr, Ni, Cu,Mo, Nb, V, Ta, W, Sn, Sb, and As and the presence of a predeterminedamount of solute B in a surface layer provide a steel that achieves highhardenability. However, Patent Literature 4 specifies the hydrogenconcentration in an atmosphere in the annealing step as 95% or more anddoes not describe whether nitrogen absorption can be suppressed toensure solute B in an annealing step in a nitrogen atmosphere.

According to the techniques described in Patent Literatures 5 to 7, theincorporation of B and one or more of Sb, Sn, Bi, Ge, Te, and Se in anamount of 0.002% to 0.03% in total is highly effective in preventingnitrogen infiltration, and, for example, even when annealing isperformed in a nitrogen atmosphere, nitrogen infiltration is prevented,and a predetermined amount of solute B is maintained, thus enhancinghardenability. However, none of Patent Literatures 5 to 7 describe thequenching hardness in a surface layer.

According to the technique described in Patent Literature 8, a steelthat contains C: 0.15% to 0.37%, B, and at least one of Sb and Sn andhence has high hardenability is proposed. However, Patent Literature 8does not discuss higher hardenability, such as carburizinghardenability.

Aspects of the present invention have been made in view of the foregoingproblems, and it is an object according to aspects of the presentinvention to provide a high-carbon hot-rolled steel sheet having highcold workability and high hardenability (immersion-quench hardenabilityand carburizing hardenability) and a method for manufacturing thehigh-carbon hot-rolled steel sheet.

To achieve the above object, the present inventors have conductedintensive studies on the relationship among conditions for theproduction of a high-carbon hot-rolled steel sheet having a steelchemical composition containing B and one or two selected from Sn andSb, cold workability, and hardenability (immersion-quench hardenabilityand carburizing hardenability) and obtained the following findings.

i) When annealing is performed in a nitrogen atmosphere, nitrogen in theatmosphere is infiltrated and concentrated into a steel sheet and bindsto B and Al in the steel sheet to form boron nitride and aluminumnitride in a surface layer. This may reduce the amount of solute B inthe steel sheet, or the presence of aluminum nitride may decrease theaustenite grain size during heating in the austenite range beforequenching, thus resulting in insufficient quenching. Thus, in accordancewith aspects of the present invention, when annealing is performed in anitrogen atmosphere, at least one of Sb and Sn is added in apredetermined amount into a steel sheet required to have higherhardenability (high carburizing hardenability). In addition, in theannealing, heating is performed at a predetermined heating rate in atemperature range from 450° C. to 600° C., whereby the amount ofnitrogen infiltration from the atmosphere into the steel can be reducedto a predetermined amount. As a result, the above nitrogen infiltrationis prevented, and a decrease in the amount of solute B and an increasein aluminum nitride are suppressed, so that higher hardenability (highcarburizing hardenability) can be ensured.

ii) The cold workability, and the degree of hardness (hardness) and thetotal elongation (hereinafter also referred to simply as elongation) ofa high-carbon hot-rolled steel sheet before quenching are greatlyinfluenced by cementite grains having an equivalent circle diameter of0.1 μm or less. When the proportion of the number of cementite grainshaving an equivalent circle diameter of 0.1 μm or less to the totalnumber of cementite grains is 20% or less, a tensile strength of 480 MPaor less and a total elongation (El) of 33% or more can be achieved.

iii) The degree of hardness (hardness) and the total elongation of ahigh-carbon hot-rolled steel sheet before quenching are greatlyinfluenced by cementite grains having an equivalent circle diameter of0.1 μm or less. When the proportion of the number of cementite grainshaving an equivalent circle diameter of 0.1 μm or less to the totalnumber of cementite grains is 10% or less, a tensile strength of 440 MPaor less and a total elongation (El) of 36% or more can be achieved.

iv) A desired microstructure can be ensured as follows: after hot roughrolling, finish rolling is performed at a finishing temperature equal toor higher than an Ar₃ transformation temperature, and then cooling isperformed to 650° C. to 750° C. at an average cooling rate of 20° C./secto 100° C./sec; coiling is performed at a coiling temperature of 500° C.to 700° C., and the coil is cooled to normal temperature to obtain ahot-rolled steel sheet; the hot-rolled steel sheet is then heatedbetween 450° C. and 600° C. at an average heating rate of 15° C./h ormore; and annealing that involves holding at an annealing temperaturelower than an Ac₁ transformation temperature for 1.0 h or more isperformed.

v) Alternatively, the desired microstructure can be ensured as follows:after hot rough rolling, finish rolling is performed at a finishingtemperature equal to or higher than an Ar₃ transformation temperature,and then cooling is performed to 650° C. to 750° C. at an averagecooling rate of 20° C./sec to 100° C./sec; coiling is performed at acoiling temperature of 500° C. to 700° C., and the coil is cooled tonormal temperature to obtain a hot-rolled steel sheet; the hot-rolledsteel sheet is then heated between 450° C. and 600° C. at an averageheating rate of 15° C./h or more; and two-stage annealing that involvesholding at a temperature equal to or higher than an Ac₁ transformationtemperature and equal to or lower than an Ac₃ transformation temperaturefor 0.5 h or more, followed by cooling to a temperature lower than anAr₁ transformation temperature at an average cooling rate of 1° C./h to20° C./h, and holding at a temperature lower than the Ar₁ transformationtemperature for 20 h or more is performed.

Aspects of the present invention are based on these findings, and are asfollows.

[1] A high-carbon hot-rolled steel sheet has a chemical compositioncontaining, by mass %, C: 0.20% or more and 0.50% or less, Si: 0.8% orless, Mn: 0.10% or more and 0.80% or less, P: 0.03% or less, S: 0.010%or less, sol. Al: 0.10% or less, N: 0.01% or less, Cr: 1.0% or less, B:0.0005% or more and 0.005% or less, and one or two selected from Sb andSn in an amount of 0.002% or more and 0.1% or less in total, with thebalance being Fe and unavoidable impurities. The steel sheet has amicrostructure including ferrite, cementite, and pearlite that accountsfor 6.5% or less of the entire microstructure by area fraction.Regarding the cementite, the proportion of the number of cementitegrains having an equivalent circle diameter of 0.1 μm or less to thetotal number of cementite grains is 20% or less, the average cementitegrain size is 2.5 μm or less, and the cementite accounts for 3.5% ormore and 10.0% or less of the entire microstructure by area fraction.The average concentration of solute B in a region extending from asurface layer to a depth of 100 μm is 10 mass ppm or more. The averageconcentration of N present as AlN in the region extending from thesurface layer to the depth of 100 μm is 70 mass ppm or less.[2] The high-carbon hot-rolled steel sheet according to [1] has atensile strength of 480 MPa or less and a total elongation of 33% ormore.[3] In the high-carbon hot-rolled steel sheet according to [1] or [2],the ferrite has an average grain size of 4 to 25 μm.[4] In the high-carbon hot-rolled steel sheet according to any one of[1] to [3], the chemical composition further contains, by mass %, one ortwo groups selected from Group A and Group B.

Group A: Ti: 0.06% or less

Group B: one or two or more selected from Nb, Mo, Ta, Ni, Cu, V, and Weach in an amount of 0.0005% or more and 0.1% or less

[5] A method for manufacturing the high-carbon hot-rolled steel sheetaccording to any one of [1] to [4] includes subjecting a steel havingthe chemical composition to hot rough rolling and then performing finishrolling at a finishing temperature equal to or higher than an Ar₃transformation temperature; then performing cooling to 650° C. to 750°C. at an average cooling rate of 20° C./sec to 100° C./sec; performingcoiling at a coiling temperature of 500° C. to 700° C. to obtain ahot-rolled steel sheet; then heating the hot-rolled steel sheet in atemperature range from 450° C. to 600° C. at an average heating rate of15° C./h or more; and performing annealing that involves holding at anannealing temperature lower than an Ac₁ transformation temperature for1.0 h or more.[6] A method for manufacturing the high-carbon hot-rolled steel sheetaccording to any one of [1] to [4] includes subjecting a steel havingthe chemical composition to hot rough rolling and then performing finishrolling at a finishing temperature equal to or higher than an Ar₃transformation temperature; then performing cooling to 650° C. to 750°C. at an average cooling rate of 20° C./sec to 100° C./sec; performingcoiling at a coiling temperature of 500° C. to 700° C. to obtain ahot-rolled steel sheet; then heating the hot-rolled steel sheet to atemperature range from 450° C. to 600° C. at an average heating rate of15° C./h or more; and performing annealing that involves holding at atemperature equal to or higher than an Ac₁ transformation temperatureand equal to or lower than an Ac₃ transformation temperature for 0.5 hor more, followed by cooling to a temperature lower than an Ar₁transformation temperature at an average cooling rate of 1° C./h to 20°C./h, and holding at a temperature lower than the Ar₁ transformationtemperature for 20 h or more.

According to aspects of the present invention, a high-carbon hot-rolledsteel sheet having high cold workability and high hardenability(immersion-quench hardenability and carburizing hardenability) isprovided. The use of the high-carbon hot-rolled steel sheet manufacturedaccording to aspects of the present invention as a material steel sheetrequired to have cold workability for automotive parts such as seatrecliners, door latches, and driving systems can contributesignificantly to the production of automotive parts required to havestable quality, thus producing industrially excellent effects.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Hereinafter, a high-carbon hot-rolled steel sheet according to aspectsof the present invention and a method for manufacturing the high-carbonhot-rolled steel sheet will be described in detail. The presentinvention is not limited to the following embodiments.

1) Chemical Composition

The chemical composition of the high-carbon hot-rolled steel sheetaccording to aspects of the present invention and the reason for thelimitation will be described. Unless otherwise specified, “%”, which isa unit of the content in the following chemical composition, means “mass%”.

C: 0.20% or more and 0.50% or less

C is an element important to provide the strength after quenching. Ifthe C content is less than 0.20%, a desired hardness is not provided byheat treatment after forming, and thus the C content needs to be 0.20%or more. However, a C content of more than 0.50% causes hardening,leading to deterioration of toughness and cold workability. Thus, the Ccontent is 0.20% or more and 0.50% or less. When the steel sheet is usedfor cold working of a part having a complex shape and difficult to formby pressing, the C content is preferably 0.45% or less, more preferably0.40% or less.

Si: 0.8% or Less

Si is an element that increases strength through solid-solutionstrengthening. A higher Si content results in a higher hardness todeteriorate cold workability, and thus the Si content is 0.8% or less,preferably 0.65% or less, more preferably 0.50% or less. When evenhigher cold workability is required in difficult-to-form partapplications, the Si content is preferably 0.30% or less. To ensuredesired softening resistance in the tempering step after quenching, theSi content is preferably 0.1% or more, more preferably 0.2% or more.

Mn: 0.10% or more and 0.80% or less

Mn is an element that improves hardenability and increases strengththrough solid-solution strengthening. If the Mn content is less than0.10%, both immersion-quench hardenability and carburizing hardenabilitybegin to deteriorate, and thus the Mn content is 0.10% or more. When theinner portion of a thick material or the like is to be reliablyquenched, the Mn content is preferably 0.25% or more, more preferably0.30% or more. If the Mn content exceeds 0.80%, a banded structure dueto Mn segregation develops, resulting in an inhomogeneousmicrostructure, and the steel becomes hard through solid-solutionstrengthening, resulting in low cold workability. Thus, the Mn contentis 0.80% or less. In the case of a material for a part required to haveformability, a certain level of cold workability is necessary, and thusthe Mn content is preferably 0.65% or less, more preferably 0.55% orless.

P: 0.03% or less

P is an element that increases strength through solid-solutionstrengthening. If the P content exceeds 0.03%, grain boundaryembrittlement is caused to deteriorate the toughness after quenching.The cold workability is also reduced. Thus, the P content is 0.03% orless. To provide high toughness after quenching, the P content ispreferably 0.02% or less. Since P reduces the cold workability and thetoughness after quenching, the P content is preferably as low aspossible. However, an excessive reduction in P leads to an increase inrefining cost, and thus the P content is preferably 0.005% or more, morepreferably 0.007% or more.

S: 0.010% or less

S is an element that needs to be minimized because S forms sulfides andreduces the cold workability and the toughness after quenching of thehigh-carbon hot-rolled steel sheet. If the S content exceeds 0.010%, thecold workability and the toughness after quenching of the high-carbonhot-rolled steel sheet deteriorate significantly. Thus, the S content is0.010% or less. To provide high cold workability and high toughnessafter quenching, the S content is preferably 0.005% or less. Since Sreduces the cold workability and the toughness after quenching, the Scontent is preferably as low as possible. However, an excessivereduction in S leads to an increase in refining cost, and thus the Scontent is preferably 0.0005% or more.

sol. Al: 0.10% or less

If the sol. Al content exceeds 0.10%, AlN is formed during heating inquenching treatment, resulting in excessively fine austenite grains.This promotes the formation of a ferrite phase during cooling to form amicrostructure composed of ferrite and martensite, resulting in lowhardness after quenching. Thus, the sol. Al content is 0.10% or less,preferably 0.06% or less. sol. Al has a deoxidation effect, and toachieve sufficient deoxidation, the sol. Al content is preferably 0.005%or more.

N: 0.01% or less

If the N content exceeds 0.01%, the formation of AlN leads to theformation of excessively fine austenite grains during heating inquenching treatment, which promotes the formation of a ferrite phaseduring cooling, resulting in low hardness after quenching. Thus, the Ncontent is 0.01% or less, preferably 0.0065% or less, more preferably0.0050% or less. N is an element that forms AlN, Cr-based nitride, and Bnitride and thus moderately inhibits the growth of austenite grainsduring heating in quenching treatment to improve the toughness afterquenching. Thus, the N content is preferably 0.0005% or more, morepreferably 0.0010% or more.

Cr: 1.0% or less

In accordance with aspects of the present invention, Cr is an importantelement that enhances hardenability. If the Cr content in the steel is0%, ferrite is readily formed in a surface layer particularly duringcarburizing and quenching, and a completely quenched microstructure isnot obtained, which may increase the likelihood of a decrease inhardness. Thus, when the steel sheet is used in applications wherehardenability is important, the Cr content is preferably 0.05% or more,more preferably 0.10% or more, still more preferably 0.20% or more. Ifthe Cr content exceeds 1.0%, the steel sheet before quenching becomeshard to have impaired cold workability. Thus, the Cr content is 1.0% orless. When a part difficult to form by pressing and requiring highworkability is processed, even higher cold workability is required, andthus the Cr content is preferably 0.7% or less, more preferably 0.5% orless.

B: 0.0005% or more and 0.005% or less

In accordance with aspects of the present invention, B is an importantelement that enhances hardenability. If the B content is less than0.0005%, the effect is not sufficiently produced. Thus, the B contentneeds to be 0.0005% or more, and is preferably 0.0010% or more. If the Bcontent exceeds 0.005%, the recrystallization of austenite after finishrolling is retarded to develop a texture of the hot-rolled steel sheet,resulting in high anisotropy after annealing to increase the likelihoodthat an earing occurs in drawing. Thus, the B content is 0.005% or less,preferably 0.004% or less.

Total content of one or two selected from Sn and Sb: 0.002% or more and0.1% or less

Sb and Sn are elements effective in suppressing nitrogen infiltrationthrough the steel sheet surface layer. If the total content of one ormore of these elements is less than 0.002%, the effect is notsufficiently produced. Thus, the total content of one or more of theseelements is 0.002% or more, more preferably 0.005% or more. If one ormore of these elements are contained in an amount of more than 0.1% intotal, the nitrogen infiltration prevention effect plateaus. Inaddition, these elements tend to segregate at grain boundaries, and thusif these elements are contained in an amount of more than 0.1% in total,grain boundary embrittlement may occur due to the excessively highcontent. Thus, the total content of one or two selected from Sb and Snis 0.1% or less, preferably 0.03% or less, still more preferably 0.02%or less.

In accordance with aspects of the present invention, since one or twoselected from Sb and Sn is contained in an amount of 0.002% or more and0.1% or less in total, nitrogen infiltration through the steel sheetsurface layer is suppressed even when annealing is performed in anitrogen atmosphere, and an increase in nitrogen concentration in thesteel sheet surface layer is suppressed. Thus, according to aspects ofthe present invention, nitrogen infiltration through the steel sheetsurface layer can be suppressed; therefore, even when annealing isperformed in a nitrogen atmosphere, the amount of solute B in a regionextending from the steel sheet surface layer to a depth of 100 μm afterannealing can be appropriately ensured, and the formation of aluminumnitride (AlN) in the region extending from the steel sheet surface layerto the depth of 100 μm can be suppressed to allow austenite grains togrow during heating before quenching. As a result, the formation offerrite and pearlite can be hindered during cooling, thus providing highhardenability.

In accordance with aspects of the present invention, the balance is Feand unavoidable impurities.

The above-described essential elements provide the high-carbonhot-rolled steel sheet according to aspects of the present inventionwith the desired properties. To further improve, for example,hardenability, the high-carbon hot-rolled steel sheet according toaspects of the present invention may optionally contain the followingelements.

Ti: 0.06% or less

Ti is an element effective in enhancing hardenability. When sufficienthardenability is not provided by the incorporation of B alone, thehardenability can be improved by the incorporation of Ti. This effect isnot produced when the Ti content is less than 0.005%, and thus if Ti iscontained, the Ti content is preferably 0.005% or more, more preferably0.007% or more. When the Ti content exceeds 0.06%, the steel sheetbefore quenching becomes hard to have impaired cold workability, andthus if Ti is contained, the Ti content is 0.06% or less, preferably0.04% or less.

Furthermore, to stabilize the mechanical properties and hardenabilityaccording to aspects of the present invention, one or two or moreselected from Nb, Mo, Ta, Ni, Cu, V, and W may be added each in arequired amount.

Nb: 0.0005% or more and 0.1% or less

Nb is an element that forms a carbonitride and is effective inpreventing exaggerated grain growth during heating before quenching,improving toughness, and improving temper softening resistance. When theNb content is less than 0.0005%, the effect of addition is notsufficiently produced. Thus, if Nb is contained, the lower limit ispreferably 0.0005%, more preferably 0.0010% or more. When the Nb contentexceeds 0.1%, the effect of addition plateaus, and, in addition, aniobium carbide increases the tensile strength of the base metal todecrease elongation. Thus, if Nb is contained, the upper limit ispreferably 0.1%, more preferably 0.05% or less, still more preferablyless than 0.03%.

Mo: 0.0005% or more and 0.1% or less

Mo is an element effective in improving hardenability and tempersoftening resistance. When the Mo content is less than 0.0005%, theeffect of addition is small. Thus, if Mo is contained, the lower limitis preferably 0.0005%, more preferably 0.0010% or more. When the Mocontent exceeds 0.1%, the effect of addition plateaus, and the costincreases. Thus, if Mo is contained, the upper limit is preferably 0.1%,more preferably 0.05% or less, still more preferably less than 0.03%.

Ta: 0.0005% or more and 0.1% or less

Ta is an element that forms a carbonitride similarly to Nb and iseffective in preventing exaggerated grain growth during heating beforequenching, preventing coarsening of grains, and improving tempersoftening resistance. When the Ta content is less than 0.0005%, theeffect of addition is small. Thus, if Ta is contained, the lower limitis preferably 0.0005%, more preferably 0.0010% or more. When the Tacontent exceeds 0.1%, the effect of addition plateaus, the quenchinghardness decreases due to excessive carbide formation, and the costincreases. Thus, if Ta is contained, the upper limit is preferably 0.1%,more preferably 0.05% or less, still more preferably less than 0.03%.

Ni: 0.0005% or more and 0.1% or less

Ni is an element highly effective in improving toughness andhardenability. When the Ni content is less than 0.0005%, the effect ofaddition is not produced. Thus, if Ni is contained, the lower limit ispreferably 0.0005%, more preferably 0.0010% or more. When the Ni contentexceeds 0.1%, the effect of addition plateaus, and, in addition, thecost increases. Thus, if Ni is contained, the upper limit is preferably0.1%, more preferably 0.05% or less.

Cu: 0.0005% or more and 0.1% or less

Cu is an element effective in ensuring hardenability. When the Cucontent is less than 0.0005%, the effect of addition is not sufficientlyproduced. Thus, if Cu is contained, the lower limit is preferably0.0005%, more preferably 0.0010% or more. When the Cu content exceeds0.1%, flaws are likely to occur during hot rolling, resulting in lowermanufacturability, such as lower yields. Thus, if Cu is contained, theupper limit is preferably 0.1%, more preferably 0.05% or less.

V: 0.0005% or more and 0.1% or less

V is an element that forms a carbonitride similarly to Nb and Ta and iseffective in preventing exaggerated grain growth during heating beforequenching, improving toughness, and improving temper softeningresistance. When the V content is less than 0.0005%, the effect ofaddition is not sufficiently produced. Thus, if V is contained, thelower limit is preferably 0.0005%, more preferably 0.0010% or more. Whenthe V content exceeds 0.1%, the effect of addition plateaus, and, inaddition, a niobium carbide increases the tensile strength of the basemetal to decrease elongation. Thus, if V is contained, the upper limitis preferably 0.1%, more preferably 0.05% or less, still more preferablyless than 0.03%.

W: 0.0005% or more and 0.1% or less

W is an element that forms a carbonitride similarly to Nb and V and iseffective in preventing exaggerated growth of austenite grains duringheating before quenching and improving tempering softening resistance.When the W content is less than 0.0005%, the effect of addition issmall. Thus, if W is contained, the lower limit is preferably 0.0005%,more preferably 0.0010% or more. When the W content is more than 0.1%,the effect of addition plateaus, the quenching hardness decreases due toexcessive carbide formation, and the cost increases. Thus, if W iscontained, the upper limit is preferably 0.1%, more preferably 0.05% orless, still more preferably less than 0.03%.

In accordance with aspects of the present invention, when two or moreselected from Nb, Mo, Ta, Ni, Cu, V, and W are contained, the totalcontent thereof is preferably 0.001% or more and 0.1% or less.

2) Microstructure

The reason for the limitation of the microstructure of the high-carbonhot-rolled steel sheet according to aspects of the present inventionwill be described.

In accordance with aspects of the present invention, the microstructureincludes ferrite and cementite. Regarding the cementite, the proportionof the number of cementite grains having an equivalent circle diameterof 0.1 μm or less to the total number of cementite grains is 20% orless, the average cementite grain size is 2.5 μm or less, and thecementite accounts for 3.5% or more and 10.0% or less of the entiremicrostructure by area fraction. The average concentration of solute Bin a region extending from a surface layer to a depth of 100 μm is 10mass ppm or more. The average concentration of N present as AlN in theregion extending from the surface layer to the depth of 100 μm is 70mass ppm or less.

In accordance with aspects of the present invention, the average grainsize of the ferrite is preferably 4 to 25 μm, more preferably 5 μm ormore.

2-1) Ferrite and Cementite

The microstructure of the high-carbon hot-rolled steel sheet accordingto aspects of the present invention includes ferrite and cementite. Inaccordance with aspects of the present invention, the area fraction ofthe ferrite is preferably 90% or more. A ferrite area fraction of lessthan 90% may reduce formability, thus making it difficult to performcold working in the case of a part requiring high workability. Thus, theferrite area fraction is preferably 90% or more, more preferably 92% ormore.

In the microstructure of the high-carbon hot-rolled steel sheetaccording to aspects of the present invention, pearlite may be formed inaddition to the ferrite and cementite described above. Pearlite may becontained as long as the area fraction thereof in the entiremicrostructure is 6.5% or less because pearlite in such an amount doesnot impair the advantageous effects according to aspects of the presentinvention.

2-2) Proportion of Number of Cementite Grains Having Equivalent CircleDiameter of 0.1 μm or Less to Total Number of Cementite Grains: 20% orLess

If the number of cementite grains having an equivalent circle diameterof 0.1 μm or less is large, the hardness increases through dispersionstrengthening to decrease elongation. To provide cold workability, inaccordance with aspects of the present invention, the proportion of thenumber of cementite grains having an equivalent circle diameter of 0.1μm or less to the total number of cementite grains is 20% or less. Thiscan further achieve a tensile strength of 480 MPa or less and a totalelongation (El) of 33% or more.

When the high-carbon hot-rolled steel sheet is used for adifficult-to-form part, high cold workability is required, and in thiscase, the proportion of the number of cementite grains having anequivalent circle diameter of 0.1 μm or less to the total number ofcementite grains is preferably 10% or less. When the proportion thenumber of cementite grains having an equivalent circle diameter of 0.1μm or less to the total number of cementite grains is 10% or less, atensile strength of 440 MPa or less and a total elongation (El) of 36%or more can be achieved. The reason why the proportion of cementitegrains having an equivalent circle diameter of 0.1 μm or less isspecified is that cementite grains of 0.1 μm or less have a dispersionstrengthening ability, and an increase in the number of cementite grainshaving such a size impairs cold workability.

To suppress exaggerated growth of ferrite grains during annealing, theproportion of the number of cementite grains having an equivalent circlediameter of 0.1 μm or less to the total number of cementite grains ispreferably 3% or more.

Cementite grains present before quenching have an equivalent circlediameter of about 0.07 to 3.0 μm. Thus, the dispersion state ofcementite grains before quenching having an equivalent circle diameterof more than 0.1 μm, which is a size not affecting precipitationstrengthening much, is not particularly specified in accordance withaspects of the present invention.

2-3) Average Cementite Grain Size: 2.5 μm or Less

In quenching, the cementite needs to be wholly dissolved to ensure adesired amount of solute C in the ferrite. If the average cementitegrain size exceeds 2.5 μm, the cementite cannot be completely dissolvedduring holding in the austenite range, and thus the average cementitegrain size is 2.5 μm or less, more preferably 2.0 μm or less. If thecementite is excessively fine, precipitation strengthening of thecementite reduces cold workability, and thus the average cementite grainsize is preferably 0.1 μm or more, more preferably 0.15 μm or more.

In accordance with aspects of the present invention, the term “cementitegrain size” refers to an equivalent circle diameter of a cementitegrain, and the equivalent circle diameter of a cementite grain is avalue obtained by measuring the major axis and the minor axis of thecementite grain and converting them into an equivalent circle diameter.The term “average cementite grain size” refers to a value determined bydividing the sum of equivalent circle diameters of all cementite grainsby the total number of cementite grains.

2-4) Proportion (Area Fraction) of Cementite Relative to EntireMicrostructure: 3.5% or More and 10.0% or Less

If the proportion of the cementite to the entire microstructure exceeds10.0%, the number of cementite grains of 0.1 μm or less contributing toprecipitation strengthening is also increased, and the steel becomeshard. Thus, the proportion of the cementite is 10.0% or less, preferably9.5% or less. On the other hand, if this proportion is less than 3.5%,the substantial C content does not reach 0.20%, and a desired hardnesscannot be provided after heat treatment. Thus, the proportion is 3.5% ormore, more preferably 4.0% or more.

2-5) Average Grain Size of Ferrite: 4 to 25 μm (Suitable Condition)

If the average grain size of the ferrite is less than 4 μm, the strengthbefore cold working may increase to deteriorate press formability, andthus the average grain size of the ferrite is preferably 4 μm or more.If the average grain size of the ferrite exceeds 25 μm, the strength ofthe base metal may decrease. In the field where the steel sheet isformed into an intended product shape and then used without quenching,the base metal needs to have some degree of strength. Thus, the averagegrain size of the ferrite is preferably 25 μm or less. The average grainsize of the ferrite is more preferably 5 μm or more, still morepreferably 6 μm or more, and more preferably 20 μm or less, still morepreferably 18 μm or less.

In accordance with aspects of the present invention, the equivalentcircle diameter of a cementite grain, the average cementite grain size,the proportion of the cementite to the entire microstructure, the areafraction of the ferrite, the average grain size of the ferrite, etc.described above can be measured by methods described in EXAMPLESdescribed later.

2-6) Average Concentration of Solute B in Region Extending from SurfaceLayer to Depth of 100 μm: 10 Mass Ppm or More

In the high-carbon hot-rolled steel sheet according to aspects of thepresent invention, to prevent the formation of a quenched microstructuresuch as pearlite or sorbite, which is likely to be formed in a surfacelayer portion when the steel sheet is quenched, B in a region (portion)extending from the steel sheet surface layer to a 100 μm position in thethickness direction (surface layer 100 μm portion) needs to be presentat an average concentration of 10 mass ppm or more in the form of soluteB that is not nitrided or oxidized. Automotive parts that are subjectedto quenching treatment for use and required to have wear resistance arerequired to have surface hardness. To provide a desired surfacehardness, it is necessary to form a completely quenched microstructurein the surface layer 100 μm portion after quenching. The averageconcentration of the solute B is preferably 12 mass ppm or more, morepreferably 15 mass ppm or more. An excessively high concentration of thesolute B impedes the development of an aggregation texture of hot-rolledmicrostructures, and thus the average concentration of the solute B is40 mass ppm or less, more preferably 35 mass ppm or less.

2-7) Average Concentration of N Present as AlN in Region Extending fromSurface Layer to Depth of 100 μm: 70 Mass Ppm or Less

When the average concentration of N present as AlN in the regionextending from the steel sheet surface layer to the 100 μm position inthe thickness direction is 70 mass ppm or less, the growth of grains ispromoted in the austenite range during heating before quenching. Thisreduces the likelihood of the formation of a microstructure such aspearlite or sorbite in the cooling stage and provides the desiredsurface hardness without causing insufficient quenching. The averageconcentration of N present as AlN in the region extending from thesurface layer to the depth of 100 μm is preferably 50 mass ppm or less.

To inhibit the exaggerated grain growth during heating in the austeniterange, the average concentration of N is preferably 10 mass ppm or more,more preferably 20 mass ppm or more.

In accordance with aspects of the present invention, it has been foundthat the amounts of solute B and N present as AlN in the steel sheetsurface layer portion are closely related to the manufacturingconditions in each step including heating conditions, coilingconditions, and annealing conditions and that these manufacturingconditions need to be optimized. The reasons necessary for achieving theamounts of solute B and N present as AlN in each step will be describedlater.

3) Mechanical Properties

The high-carbon hot-rolled steel sheet according to aspects of thepresent invention is used to form automotive parts such as gears,transmissions, and seat recliners by cold pressing and thus is requiredto have high cold workability. In addition, it is necessary to impartwear resistance by increasing the hardness through quenching treatment.Thus, the high-carbon hot-rolled steel sheet according to aspects of thepresent invention has a reduced tensile strength (TS) of 480 MPa or lessand an increased total elongation (El) of 33% or more and hence canachieve both high cold workability and high hardenability(immersion-quench hardenability and carburizing hardenability). Morepreferably, the TS is 460 MPa or less, and the El is 35% or more.

In the case where the steel sheet is used to form a difficult-to-formpart required to have cold pressing properties, the tensile strength ofthe steel sheet is further reduced to a TS of 440 MPa or less, and thetotal elongation of the steel sheet is further increased to an El of 36%or more, whereby both high cold workability and high hardenability(immersion-quench hardenability and carburizing hardenability) can beachieved. More preferably, the TS is 410 MPa or less, and the El is 38%or more.

The tensile strength (TS) and the total elongation (El) described abovecan be measured by methods described in EXAMPLES described later.

4) Manufacturing Method

The high-carbon hot-rolled steel sheet according to aspects of thepresent invention is manufactured in the following manner using, as amaterial, a steel having a chemical composition as described above. Thematerial (steel material) is subjected to hot rough rolling, and thenfinish rolling is performed at a finishing temperature equal to orhigher than an Ar₃ transformation temperature. Subsequently, cooling isperformed to 650° C. to 750° C. at an average cooling rate of 20° C./secto 100° C./sec. Coiling is performed at a coiling temperature of 500° C.to 700° C., and the coil is cooled to normal temperature to obtain ahot-rolled steel sheet. The hot-rolled steel sheet is then heated in atemperature range from 450° C. to 600° C. at an average heating rate of15° C./h or more. Annealing that involves holding at an annealingtemperature lower than an Ac₁ transformation temperature for 1.0 h ormore is performed.

Alternatively, the high-carbon hot-rolled steel sheet according toaspects of the present invention is manufactured in the following mannerusing, as a material, a steel having a chemical composition as describedabove. The material (steel material) is subjected to hot rough rolling,and then finish rolling is performed at a finishing temperature equal toor higher than an Ar₃ transformation temperature. Subsequently, coolingis performed to 650° C. to 750° C. at an average cooling rate of 20°C./sec to 100° C./sec. Coiling is performed at a coiling temperature of500° C. to 700° C., and the coil is cooled to normal temperature toobtain a hot-rolled steel sheet. The hot-rolled steel sheet is thenheated in a temperature range from 450° C. to 600° C. at an averageheating rate of 15° C./h or more. Two-stage annealing that involvesholding at a temperature equal to or higher than an Ac₁ transformationtemperature and equal to or lower than an Ac₃ transformation temperaturefor 0.5 h or more, followed by cooling to a temperature lower than anAr₁ transformation temperature at an average cooling rate of 1° C./h to20° C./h, and holding at a temperature lower than the Ar₁ transformationtemperature for 20 h or more is performed.

Hereinafter, the reason for the limitation in the method formanufacturing the high-carbon hot-rolled steel sheet according toaspects of the present invention will be described. In the description,the expression “° C.” regarding temperature indicates a temperature at asteel sheet surface or a surface of a steel material.

In accordance with aspects of the present invention, the steel materialmay be produced by any method. For example, to prepare a moltenhigh-carbon steel according to aspects of the present invention, eithera converter or an electric furnace can be used. The molten high-carbonsteel prepared by a known method, for example, using a converter isformed into, for example, a slab (steel material) by ingot making andblooming or continuous casting. Typically, the slab is heated and thensubjected to hot rolling (hot rough rolling and finish rolling).

For example, in the case of a slab produced by continuous casting,direct rolling in which the slab is rolled as it is or while being kepthot for the purpose of suppressing temperature drop may be used. Whenthe slab is heated and subjected to hot rolling, the heating temperatureof the slab is preferably 1280° C. or lower in order to avoiddeterioration of the surface state due to scales. The lower limit of theheating temperature of the slab is preferably 1100° C., more preferably1150° C., still more preferably 1200° C. or higher. During the hotrolling, the material to be rolled may be heated by heating means suchas a sheet bar heater in order to ensure the finishing temperature.

Finish rolling at finishing temperature equal to or higher than Ar₃transformation temperature

If the finishing temperature is lower than the Ar₃ transformationtemperature, coarse ferrite grains are formed after the hot rolling andafter annealing to significantly decrease elongation. Thus, thefinishing temperature is equal to or higher than the Ar₃ transformationtemperature, preferably equal to or higher than (Ar₃ transformationtemperature+20° C.). The upper limit of the finishing temperature neednot be particularly specified, and is preferably 1000° C. or lower tosmoothly perform the cooling after the finish rolling.

The Ar₃ transformation temperature described above can be determined byactual measurement such as thermal expansion measurement or electricalresistance measurement during cooling using, for example, Formastertesting.

After finish rolling, cooling to 650° C. to 750° C. at average coolingrate of 20° C./sec to 100° C./sec

After the finish rolling, the average rate cooling to 650° C. to 750° C.greatly affects the size of spheroidized cementite grains afterannealing. If the average cooling rate after the finish rolling is lessthan 20° C./sec, a microstructure before annealing is composed of anexcessive ferrite microstructure and a pearlite microstructure, and thusa desired cementite dispersion state and a desired cementite size arenot provided after annealing. Thus, the cooling needs to be performed at20° C./sec or more. The average cooling rate is preferably 25° C./sec ormore. If the average cooling rate exceeds 100° C./sec, cementite grainshaving a desired size are not readily formed after annealing, and thusthe average cooling rate is 100° C./sec or less, preferably 75° C./secor less.

Coiling temperature: 500° C. to 700° C.

The hot-rolled steel sheet after the finish rolling is wound into a coilshape. If the coiling temperature is excessively high, the hot-rolledsteel sheet has excessively low strength and may be deformed by its ownweight when wound into a coil shape. This is not preferable from theviewpoint of operation. Thus, the upper limit of the coiling temperatureis 700° C., preferably 690° C. or lower. If the coiling temperature isexcessively low, the hot-rolled steel sheet disadvantageously becomeshard. Thus, the coiling temperature is 500° C., preferably 530° C. orhigher.

After being wound into a coil shape, the coil may be cooled to normaltemperature and subjected to pickling treatment. After the picklingtreatment, annealing is performed. For the pickling treatment, a knownmethod can be used. Subsequently, the resulting hot-rolled steel sheetis subjected to the following annealing.

Average heating rate in temperature range from 450° C. to 600° C.: 15°C./h or more

The hot-rolled steel sheet obtained as described above is subjected toannealing (spheroidizing annealing of cementite). In the case ofannealing in a nitrogen atmosphere, ammonia gas is likely to occur in atemperature range from 450° C. to 600° C., and nitrogen decomposed fromthe ammonia gas enters the surface of the steel sheet and binds to B andAl in the steel to form nitrides. Thus, the heating time in thetemperature range from 450° C. to 600° C. is set to be as short aspossible. The average heating rate in this temperature range is 15° C./hor more. To reduce variation in temperature in the furnace for thepurpose of improvement in productivity, the average heating rate ispreferably 100° C./h or less, more preferably 70° C./h or less.

Holding at annealing temperature lower than Ac₁ transformationtemperature for 1.0 h or more

If the annealing temperature is not lower than the Ac₁ transformationtemperature, austenite is precipitated, and a coarse pearlitemicrostructure is formed during the cooling process after the annealing,resulting in an inhomogeneous microstructure. Thus, the annealingtemperature is lower than the Ac₁ transformation temperature, preferably(Ac₁ transformation temperature−10° C.) or lower. The lower limit of theannealing temperature is not particularly specified, and to provide adesired cementite dispersion state, the annealing temperature ispreferably 600° C. or higher, more preferably 700° C. or higher. As anatmospheric gas, any of nitrogen, hydrogen, and a gas mixture ofnitrogen and hydrogen can be used. The holding time at the annealingtemperature is 1.0 hour (h) or more. If the holding time at theannealing temperature is less than 1.0 hour, the effect of annealing isslight, and the target microstructure according to aspects of thepresent invention is not formed, as a result of which the targethardness and elongation of the steel sheet according to aspects of thepresent invention are not provided. Thus, the holding time at theannealing temperature is 1.0 hour or more, preferably 5 hours or more,more preferably more than 20 hours. If the holding time at the annealingtemperature exceeds 40.0 hours, the productivity decreases, resulting inan excessively high manufacturing cost. Thus, the holding time at theannealing temperature is preferably 40.0 hours or less, more preferably35 hours or less.

In accordance with aspects of the present invention, the followingtwo-stage annealing may be performed instead of the above-describedannealing. Specifically, the high-carbon hot-rolled steel sheet can alsobe manufactured as follows: after coiling and cooling to normaltemperature are performed to obtain a hot-rolled steel sheet, thehot-rolled steel sheet is heated in a temperature range from 450° C. to600° C. at an average heating rate of 15° C./h or more, and two-stageannealing that involves holding at a temperature equal to or higher thanthe Ac₁ transformation temperature and equal to or lower than the Ac₃transformation temperature for 0.5 h or more (first-stage annealing),followed by cooling to a temperature lower than an Ar₁ transformationtemperature at an average cooling rate of 1° C./h to 20° C./h, andholding at a temperature lower than the Ar₁ transformation temperaturefor 20 h or more (second-stage annealing) is performed.

In accordance with aspects of the present invention, the hot-rolledsteel sheet is heated in a temperature range from 450° C. to 600° C. atan average heating rate of 15° C./h or more, held at a temperature equalto or higher than the Ac₁ transformation temperature for 0.5 h or moreto dissolve relatively fine carbide precipitated in the hot-rolled steelsheet into a γ phase, and then cooled to a temperature lower than theAr₁ transformation temperature at an average cooling rate of 1° C./h to20° C./h and held at a temperature lower than the Ar₁ transformationtemperature for 20 h or more. This allows solute C to precipitate withrelatively coarse undissolved carbide and the like serving as nuclei toachieve a state in which the dispersion of carbide (cementite) iscontrolled such that the proportion of the number of cementite grainshaving an equivalent circle diameter of 0.1 μm or less to the totalnumber of cementite grains is 20% or less. That is to say, in accordancewith aspects of the present invention, the dispersion morphology ofcarbide is controlled by performing the two-stage annealing under thepredetermined conditions, whereby the steel sheet is softened. For thesoftening of the high-carbon steel sheet of interest in accordance withaspects of the present invention, it is important to control thedispersion morphology of carbide after the annealing. In accordance withaspects of the present invention, the high-carbon hot-rolled steel sheetis held at a temperature equal to or higher than the Ac₁ transformationtemperature and equal to or lower than the Ac₃ transformationtemperature (first-stage annealing), whereby fine carbide is dissolved,and at the same time, C is dissolved in γ (austenite). In the subsequentcooling to a temperature lower than the Ar₁ transformation temperatureand holding (second-stage annealing), the α/γ interface and undissolvedcarbide present in a temperature range of the Ac₁ transformationtemperature or higher serve as nucleation sites to precipitaterelatively coarse carbide. The conditions for the two-stage annealingwill be described below. As an atmospheric gas during the annealing, anyof nitrogen, hydrogen, and a gas mixture of nitrogen and hydrogen can beused.

Average heating rate in temperature range from 450° C. to 600° C.: 15°C./h or more

For the same reasons as above, ammonia gas is likely to occur in atemperature range from 450° C. to 600° C., and nitrogen decomposed fromthe ammonia gas enters the surface of the steel sheet and binds to B andAl in the steel to form nitrides. Thus, the heating time in thetemperature range from 450° C. to 600° C. is set to be as short aspossible. The average heating rate in this temperature range is 15° C./hor more, preferably 20° C./h or more. The upper limit of the averageheating rate is preferably 100° C./h, more preferably 90° C./h or less.

Holding at temperature equal to or higher than Ac₁ transformationtemperature and equal to or lower than Ac₃ transformation temperaturefor 0.5 h or more (first-stage annealing)

By holding the hot-rolled steel sheet at a temperature equal to orhigher than the Ac₁ transformation temperature, part of ferrite in themicrostructure of the steel sheet is transformed into austenite, so thatfine carbide precipitated in ferrite is dissolved, and C is dissolved inaustenite. On the other hand, ferrite remained without being transformedinto austenite is annealed at a high temperature, and as a result, theferrite has a reduced dislocation density and softens. Undissolvedrelatively coarse carbide (undissolved carbide) remains in ferrite andbecomes further coarsened through Ostwald ripening. If the annealingtemperature is lower than the Ac₁ transformation temperature, austenitetransformation does not occur, and thus carbide cannot be dissolved inaustenite. If the first-stage annealing temperature is higher than theAc₃ transformation temperature, a large number of rod-like cementitegrains are formed after the annealing, and the desired elongation is notprovided. Thus, the first-stage annealing temperature is equal to orlower than the Ac₃ transformation temperature. In accordance withaspects of the present invention, if the holding time at a temperatureequal to or higher than the Ac₁ transformation temperature and equal toor lower than the Ac₃ transformation temperature is less than 0.5 h,fine carbide cannot be sufficiently dissolved. Thus, in the first-stageannealing, the steel sheet is held at a temperature equal to or higherthan the Ac₁ transformation temperature and equal to or lower than theAc₃ transformation temperature for 0.5 h or more. The holding time ispreferably 1.0 h or more. The holding time is preferably 10 h or less.Even when the annealing is performed while holding the steel sheet at atemperature equal to or higher than the Ac₁ transformation temperatureand equal to or lower than the Ac₃ transformation temperature, theheating rate is preferably such that the average heating rate in thetemperature range from 450° C. to 600° C. is 15° C./h or more and theupper limit is 100° C./h or less.

Cooling to temperature lower than Ar₁ transformation temperature ataverage cooling rate of 1° C./h to 20° C./h

After the first-stage annealing described above, the steel sheet iscooled to a temperature lower than the Ar₁ transformation temperaturewithin the temperature range of the second-stage annealing at an averagecooling rate of 1° C./h to 20° C./h. During the cooling, C ejected fromaustenite as a result of transformation from austenite to ferrite isprecipitated in the form of relatively coarse spherical carbide with theα/γ interface and undissolved carbide serving as nucleation sites. Inthis cooling, the cooling rate needs to be adjusted so as not to formpearlite. If the average cooling rate after the first-stage annealingand before the second-stage annealing is less than 1° C./h, theproduction efficiency is low. Thus, the average cooling rate is 1° C./hor more, preferably 5° C./h or more. If the average cooling rate exceeds20° C./h, pearlite is precipitated to increase the hardness. Thus, theaverage cooling rate is 20° C./h or less, preferably 15° C./h or less.

Holding at temperature lower than Ar₁ transformation temperature for 20h or more (second-stage annealing)

After the first-stage annealing described above, the steel sheet iscooled at a predetermined average cooling rate and held at a temperaturelower than the Ar₁ transformation temperature to cause Ostwald ripeningso that the coarse spherical carbide is further grown and fine carbidedisappears. If the holding time at a temperature lower than the Ar₁transformation temperature is less than 20 h, carbide cannot besufficiently grown, resulting in an excessively high hardness after theannealing. Thus, in the second-stage annealing, the steel sheet is heldat a temperature lower than the Ar₁ transformation temperature for 20 hor more. For sufficient growth of carbide, the second-stage annealingtemperature is preferably, but not necessarily, 660° C. or higher. Fromthe viewpoint of production efficiency, the holding time is preferably,but not necessarily, 30 h or less.

The Ac₃ transformation temperature, the Ac₁ transformation temperature,the Ar₃ transformation temperature, and the Ar₁ transformationtemperature described above can be determined by actual measurement suchas thermal expansion measurement or electrical resistance measurementduring heating or cooling using, for example, Formaster testing.

The average heating rates and the average cooling rates described aboveare determined by measuring temperatures with a thermocouple mounted inthe furnace.

EXAMPLES

Molten steels having chemical compositions of steel Nos. A to T shown inTable 1 were cast into slab, and hot rolling was then performed undermanufacturing conditions shown in Table 2-1 and Table 3-1. Subsequently,pickling was performed, and annealing (spheroidizing annealing) wasperformed in a nitrogen atmosphere (atmospheric gas: nitrogen) atannealing temperatures for annealing times (h) shown in Table 2-1 andTable 3-1 to manufacture hot-rolled annealed sheets having a thicknessof 3.0 mm.

In Examples of the present invention, test pieces were taken from thehot-rolled annealed sheets thus obtained, and the microstructure, theamount of solute B, the amount of N in AlN, the tensile strength, thetotal elongation, and the quenching hardness (hardness of steel sheetafter quenching and hardness of steel sheet after carburizing andquenching) were determined as described below. The Ac₃ transformationtemperature, the Ac₁ transformation temperature, the Ar₁ transformationtemperature, and the Ar₃ transformation temperature shown in Table 1were determined by Formaster testing.

(1) Microstructure

The microstructure of each annealed steel sheet was determined asfollows: a test piece (size: 3 mm thick×10 mm×10 mm) taken from acentral portion in the width direction was cut, polished, and thensubjected to nital etching. Images were captured with a scanningelectron microscope (SEM) at a magnification of 3000 times at fivepoints at ¼ from a surface layer in the thickness direction. Thecaptured microstructure images were subjected to image processing toidentify phases (e.g., ferrite, cementite, and pearlite). In Table 2-2and Table 3-2, “pearlite area fraction” is shown as a microstructure,and steels observed to have a pearlite area fraction of more than 6.5%are represented as Comparative Examples. Steels including pearlite withan area fraction of 6.5% or less, ferrite, and cementite are representedas Examples.

The SEM images were binarized into ferrite and a non-ferrite regionusing image analysis software to determine the area fraction (%) offerrite. Also for cementite, the SEM images were binarized intocementite and a non-cementite region using image analysis software todetermine the area fraction (%) of cementite. For pearlite, the areafractions (%) of ferrite and cementite were subtracted from 100(%) todetermine the area fraction (%) of pearlite.

In the captured microstructure images, the size of each cementite grainwas determined. The cementite grain size was determined by measuring themajor axis and the minor axis and converting them into an equivalentcircle diameter. The average cementite grain size was determined bydividing the sum of equivalent circle diameters of all cementite grainsby the total number of cementite grains. The number of cementite grainswhose equivalent circle diameter values were 0.1 μm or less wasdetermined and defined as the number of cementite grains having anequivalent circle diameter of 0.1 μm or less. The number of allcementite grains was determined and defined as the total number ofcementite grains. The proportion of the number of cementite grainshaving an equivalent circle diameter of 0.1 μm or less to the totalnumber of cementite grains ((the number of cementite grains having anequivalent circle diameter of 0.1 μm or less/the total number ofcementite grains)×100(%)) was determined. “The proportion of cementitegrains having an equivalent circle diameter of 0.1 μm or less” may alsobe referred to simply as cementite grains having an equivalent circlediameter of 0.1 μm or less.

In the captured microstructure images, the average grain size of ferritewas determined using a method for evaluation of crystal grain size(intercept method) specified in JIS G 0551.

(2) Measurement of Average Concentration of Solute B

The same method as described in the following reference was used.Specifically, ground powder from a region extending from a surface layerto a depth of 100 μm was collected and measured three times, and theaverage value was determined as the average concentration of solute B.

-   [Reference] Satoshi Kinoshiro, Tomoharu Ishida, Kunio Inose, and    Kyoko Fujimoto, Tetsu-to-Hagane (Iron and Steel), vol. 99 (2013) No.    5, p. 362-365

(3) Measurement of Average Concentration of N Present as AlN

Similarly to the above, the average concentration of N present as AlNwas determined by the same method as described in the followingreference.

-   [Reference] Satoshi Kinoshiro, Tomoharu Ishida, Kunio Inose, and    Kyoko Fujimoto, Tetsu-to-Hagane (Iron and Steel), vol. 99 (2013) No.    5, p. 362-365

(4) Tensile Strength and Elongation of Steel Sheet

Using a JIS No. 5 tensile test piece cut out from each annealed steelsheet (original sheet) in a direction at 0° with respect to the rollingdirection (L direction), a tensile test was performed at 10 mm/min. Anominal stress-nominal strain curve was determined, and the maximumstress was used as a tensile strength. The broken samples were buttedagainst each other to determine the total elongation. The result wasused as an elongation (El).

(5) Hardness of Steel Sheet after Quenching (Immersion-QuenchHardenability)

A flat test piece (15 mm wide×40 mm long×3 mm thick) was taken from acentral portion in the width direction of each annealed steel sheet, andsubjected to quenching treatment with oil cooling at 70° C. as describedbelow to determine the quenching hardness (immersion-quenchhardenability). The quenching treatment was performed in a manner thatthe flat test piece was held at 900° C. for 600 s and immediately cooledwith oil at 70° C. (70° C. oil cooling). The quenching hardness wasdetermined as follows: in a cut surface of the quenching-treated testpiece, the hardness was measured in an inner region 70 μm from thesurface layer in the width direction and at ¼ from the surface layer inthe width direction each at five points with a Vickers hardness testerunder a load to 0.2 kgf, and the average hardness was determined as thequenching hardness (HV). The above-described inner region 70 μm from thesurface layer in the thickness direction is expressed as “surface layer”in Table 2-2 and Table 3-2.

(6) Hardness of Steel Sheet after Carburizing and Quenching (CarburizingHardenability)

Each annealed steel sheet was subjected to a carburizing and quenchingtreatment including steel soaking, carburizing treatment, and diffusiontreatment at 930° C. for 4 hours in total, held at 850° C. for 30minutes, and then cooled in oil (oil cooling temperature: 60° C.). Thehardness was measured under a load of 1 kgf from a position 0.1 mm deepfrom the steel sheet surface to a position 1.2 mm deep at intervals of0.1 mm to determine the hardness (HV) at 0.1 mm from the surface layerand the effective case depth (mm) after carburizing and quenching. Theeffective case depth is defined as a depth at which the hardnessmeasured from the surface after the heat treatment reaches 550 HV ormore.

From the results obtained from the above (5) and (6), the hardenabilitywas evaluated under conditions shown in Table 4. Table 4 presentsacceptance criteria of hardenability depending on the C content, inwhich the hardenability can be evaluated as sufficient. When all of thehardness (HV) after 70° C. oil cooling, the hardness (HV) at 0.1 mm deepfrom the surface layer after carburizing and quenching, and theeffective case depth after carburizing and quenching satisfied thecriteria in Table 4, the steel sheet was judged as acceptable (denotedby the symbol ◯) and evaluated as having high hardenability. When any ofthe values did not satisfy the criteria shown in Table 4, the steelsheet was judged as unacceptable (denoted by the symbol x) and evaluatedas having poor hardenability.

TABLE 1 Chemical composition (mass %) Steel sol. No. C Si Mn P S Al N CrB Sb, Sn Ti Nb A 0.20 0.01 0.55 0.02 0.004 0.030 0.0044 0.50 0.0030 Sb +Sn:0.010 — — B 0.22 0.03 0.45 0.01 0.003 0.050 0.0041 0.40 0.0030Sb:0.012 — — C 0.28 0.79 0.50 0.02 0.004 0.010 0.0044 0.12 0.0020Sb:0.025 — — D 0.24 0.64 0.60 0.02 0.004 0.010 0.0044 0.65 0.0015 Sb +Sn:0.020 — 0.001 E 0.23 0.85 0.40 0.02 0.004 0.010 0.0044 0.50 0.0025Sb:0.010 — 0.001 F 0.22 0.15 0.85 0.02 0.004 0.050 0.0050 0.30 0.0035Sb + Sn:0.010 — — G 0.22 0.30 0.40 0.01 0.003 0.006 0.0045 0.02 0.0030Sb:0.012 — — H 0.23 0.35 0.45 0.01 0.003 0.030 0.0050 0.40 0.0032Sb:0.008 0.02 — I 0.22 0.01 0.35 0.01 0.003 0.050 0.0050 1.05 0.0020Sb + Sn:0.010 — — J 0.23 0.01 0.45 0.01 0.003 0.030 0.0050 0.40 0.00320.000 — — K 0.35 0.01 0.35 0.02 0.004 0.010 0.0044 0.01 0.0020 Sb:0.009— — L 0.33 0.15 0.31 0.01 0.003 0.120 0.0110 0.10 0.0015 Sb:0.025 0.01 —M 0.35 0.05 0.35 0.02 0.004 0.020 0.0044 0.20 0.0001 Sb + Sn:0.012 — — N0.36 0.01 0.04 0.02 0.003 0.050 0.0047 0.20 0.0030 Sb + Sn:0.011 — — O0.45 0.03 0.35 0.02 0.004 0.030 0.0050 0.02 0.0026 Sb + Sn:0.100 — — P0.48 0.25 0.31 0.01 0.004 0.010 0.0044 0.06 0.0025 Sb:0.009 — — Q 0.480.03 0.28 0.01 0.002 0.035 0.0052 0.45 0.0010 Sb + Sn:0.012 — — R 0.440.10 0.32 0.01 0.003 0.040 0.0047 0.05 0.0048 Sb:0.011 0.04 — S 0.180.05 0.45 0.01 0.004 0.035 0.0050 0.40 0.0030 Sb:0.010 0.04 — T 0.550.25 0.45 0.01 0.003 0.040 0.0050 0.45 0.0020 Sb + Sn:0.008 — — Ac₁ Ar₁Ac₃ Ar₃ transfor- transfor- transfor- transfor- mation mation mationmation temper- temper- temper- temper- Steel Chemical composition (mass%) ature ature ature ature No. Mo Ta Ni Cu V W (° C.) (° C.) (° C.) (°C.) Remarks A — — — — — — 726 716 836 824 Inventive Steel B — — — — — —726 716 835 825 Inventive Steel C — — — — — — 743 737 852 840 InventiveSteel D — — — — — — 746 736 842 832 Inventive Steel E — — — — — — 752742 861 851 Comparative Steel F — — — — — — 723 713 829 827 ComparativeSteel G — — — — — — 728 718 835 825 Inventive Steel H — — — — — — 735725 839 829 Inventive Steel I — — — — — — 737 727 832 820 ComparativeSteel J — — — — — — 725 727 824 814 Comparative Steel K 0.0015 — 0.001 —— — 720 716 810 798 Inventive Steel L — — — — — — 726 716 855 845Comparative Example M — — — — — — 724 714 811 801 Comparative Steel N —— — 0.0150 — — 726 717 830 819 Comparative Steel O — 0.0020 — 0.0015 — —720 723 801 790 Inventive Steel P — — 0.025 — 0.0015 — 728 718 794 782Inventive Steel Q — — — — — 0.0015 728 716 788 778 Inventive Steel R —0.0015 — — — — 723 713 803 793 Inventive Steel S — — — — — — 726 715 840829 Comparative Steel T 0.0012 — — — — — 733 722 785 775 ComparativeSteel

TABLE 2-1 Hot rolling conditions Annealing conditions Average Averagecooling heating rate rate to 650° C. in temper- Finish- to 750° C. atureing after Coiling range from Annealing temper- finish temper- 450° C. to(annealing Sample Steel ature rolling ature 600° C. temperature- No. No.(° C.) (° C./sec) (° C.) (° C./h) holding time)  1 A 850 40 550 45 715°C.-30 h  2 A 850 55 480 60 715° C.-30 h  3 A 850 55 680 15 715° C.-30 h 4 A 850 55 620 15 710° C.-15 h  5 B 845 30 550 30 715° C.-30 h  6 B 84530 550 30 770° C.-25 h  7 B 845 30 510 15 715° C.-30 h  8 B 845 30 620 5715° C.-30 h  9 C 860 40 620 35 715° C.-22 h 10 D 860 60 680 90 710°C.-25 h 11 E 880 50 580 20 715° C.-30 h 12 F 840 50 620 30 710° C.-25 h13 G 850 50 600 30 680° C.-25 h 14 H 850 40 580 50 715° C.-30 h 15 H 85040 510 40 715° C.-0.25 h 16 I 840 50 600 40 715° C.-30 h 17 J 830 80 70020 715° C.-30 h 18 K 830 60 700 40 715° C.-30 h 19 L 860 40 700 50 715°C.-30 h 20 M 880 50 680 60 715° C.-30 h 21 N 840 50 660 40 715° C.-30 h22 O 830 50 590 40 715° C.-30 h 23 P 830 25 550 40 715° C.-30 h 24 Q 83025 560 30 715° C.-30 h 25 R 820 40 650 45 715° C.-30 h 26 S 860 40 65040 715° C.-30 h 27 T 810 40 650 40 710° C.-25 h

TABLE 2-2 [(Number of cementite grains with equivalent Average AverageCarburizing hardenability circle Pro- concen- concen- Hardness atdiameter portion tration tration of 0.1 mm Effective of 0.1 μm of ofsolute N present Immersion-quench from surface case or less)/(totalAverage Ferrite cementite B in portion as AIN in hardenability (HV)layer after depth after Eval- number cementite average Ferrite to entirePearlite 100 μm portion Total 70° C. oil 70° C. oil carburizingcarburizing uation of cementite grain grain area micro- area fromsurface 100 μm from elon- cooling cooling and and of Sample Steel Micro-grains)] × size size fraction structure fraction layer (mass surfacelayer TS gation (surface (1/4 quenching quenching harden- No. No.structure 100 (%) (μm) (μm) (%) (area %) (%) ppm) (mass ppm) (MPa) (%)layer) thickness) (HV) (mm) ability Remarks 1 A ferrite + 15 0.40 7 953.5 1.5 15 35 460 37 360 390 670 0.70 ○ Example cementite 2 A ferrite +22 0.18 6 91 3.9 5.1 14 40 490 32 355 385 665 0.68 ○ Comparativecementite Example 3 A ferrite + 12 0.45 8 95 3.5 1.5 16 60 450 38 360395 675 0.65 ○ Example cementite 4 A ferrite + 18 0.35 4 94 3.6 2.4 1560 470 34 360 395 675 0.65 ○ Example cementite 5 B ferrite + 11 0.55 894 3.6 2.4 15 35 450 38 380 420 655 0.65 ○ Example cementite 6 Bferrite + 4 0.65 11 80 0.5 19.5 15 40 460 31 370 420 655 0.64 ○Comparative cementite + Example pearlite 7 B ferrite + 1 0.50 7 94 3.62.4 10 70 460 36 360 425 600 0.62 ○ Example cementite 8 B ferrite + 130.52 7 93 3.7 3.3 9 80 395 41 340 415 580 0.60 x Comparative cementiteExample 9 C ferrite + 14 0.45 7 93 4.7 2.3 17 40 460 35 390 470 680 0.62○ Example cementite 10 D ferrite + 12 0.50 8 94 4.0 2.0 15 30 480 33 380430 690 0.70 ○ Example cementite 11 E ferrite + 14 0.38 7 93 3.8 3.2 1440 490 32 370 425 670 0.70 ○ Comparative cementite Example 12 Fferrite + 14 0.40 6 92 3.7 4.3 14 40 490 32 390 425 700 0.80 ○Comparative cementite Example 13 G ferrite + 17 0.30 5 94 3.7 2.3 15 40430 40 370 425 650 0.65 ○ Example cementite 14 H ferrite + 12 0.50 8 923.8 4.2 12 35 430 40 370 430 700 0.70 ○ Example cementite 15 H ferrite +30 0.20 4 90 3.3 6.7 15 40 480 32 370 430 700 0.65 ○ Comparativecementite + Example pearlite 16 I ferrite + 10 0.30 8 94 3.7 2.3 15 35490 30 400 425 710 0.80 ○ Comparative cementite Example 17 J ferrite +15 0.50 7 93 3.7 3.3 5 50 430 39 300 430 580 0.65 x Comparativecementite Example 18 K ferrite + 12 0.40 8 94 5.9 0.1 12 60 420 40 510570 680 0.65 ○ Example cementite 19 L ferrite + 12 0.41 8 94 5.5 0.5 12200 420 38 380 480 580 0.65 x Comparative cementite Steel 20 M ferrite +12 0.35 8 93 5.9 1.1 0 70 430 41 470 530 570 0.55 x Comparativecementite Steel 21 N ferrite + 11 0.40 7 94 6.0 0.0 15 50 410 42 470 490590 0.50 x Comparative cementite Example 22 O ferrite + 7 0.40 9 92 7.50.5 17 40 460 35 670 680 700 0.70 ○ Example cementite 23 P ferrite + 130.45 10 92 8.0 0.0 15 35 460 36 680 700 675 0.70 ○ Example cementite 24Q ferrite + 12 0.48 9 91 8.1 0.9 14 34 450 37 680 710 675 0.70 ○ Examplecementite 25 R ferrite + 9 0.50 9 92 7.4 0.6 14 38 440 37 580 620 6950.70 ○ Example cementite 26 S ferrite + 12 0.40 7 95 2.7 2.3 15 40 40042 310 380 700 0.60 x Comparative cementite Example 27 T ferrite + 250.35 6 90 9.2 0.8 15 40 520 28 680 720 700 0.68 ○ Comparative cementiteExample

TABLE 3-1 Annealing conditions Hot rolling conditions Average AverageAverage heating rate in cooling rate cooling rate temperatureFirst-stage from first Second-stage Finishing to 650° C. to Coilingrange from annealing stage to annealing temper- 750° C. after temper-450° C. to (annealing second (annealing Sample Steel ature finishrolling ature 600° C. temperature- stage temperature- No. No. (° C.) (°C./sec) (° C.) (° C./h) holding time) (° C./h) holding time) 28 A 850 40680 50 790° C.-8 h 10 710° C.-30 h 29 A 850 55 680 50 790° C.-8 h 10710° C.-15 h 30 A 850 55 680 10 790° C.-8 h 10 710° C.-30 h 31 A 850 40680 50 820° C.-0.1 h 10 710° C.-30 h 32 B 845 30 620 40 780° C.-10 h 12710° C.-20 h 33 B 845 30 620 40 860° C.-8 h 10 710° C.-30 h 34 B 845 30670 15 800° C.-6 h 50 710° C.-30 h 35 C 860 40 680 35 790° C.-7 h 12710° C.-25 h 36 D 860 60 620 90 750° C.-8 h 10 715° C.-20 h 37 E 880 50480 30 755° C.-4 h 10 690° C.-30 h 38 F 840 50 510 30 750° C.-4 h 10690° C.-20 h 39 G 850 50 550 60 790° C.-8 h 10 710° C.-30 h 40 H 850 40510 50 760° C.-8 h 10 710° C.-25 h 41 I 840 50 600 40 770° C.-6 h 10710° C.-30 h 42 J 830 80 700 20 800° C.-6 h 10 710° C.-25 h 43 K 830 60700 15 800° C.-6 h 10 710° C.-25 h 44 L 860 40 700 50 800° C.-6 h 10710° C.-25 h 45 M 880 50 680 50 800° C.-6 h 10 710° C.-20 h 46 N 840 50660 40 790° C.-8 h 15 705° C.-30 h 47 O 830 50 590 40 790° C.-4 h 8 710°C.-25 h 48 Q 830 25 610 30 770° C.-8 h 10 710° C.-20 h 49 R 820 40 70045 800° C.-8 h 10 710° C.-30 h 50 S 860 40 650 40 810° C.-4 h 10 710°C.-21 h 51 T 810 40 650 40 800° C.-6 h 10 710° C.-25 h

TABLE 3-2 [(Number of cementite grains with equivalent Carburizinghardenability circle Pro- Hardness at diameter portion 0.1 mm Effectiveof 0.1 μm of Average Immersion-quench from surface case or less)/(totalAverage Ferrite cementite Average concen- hardenability (HV) layer afterdepth after Eval- number cementite average Ferrite to entire Pearliteconcen- tration of Total 70° C. oil 70° C. oil carburizing carburizinguation of cementite grain grain area micro- area tration N present elon-cooling cooling and and of Sample Steel Micro- grains)] × size sizefraction structure fraction of solute B as AIN TS gation (surface (1/4quenching quenching harden- No. No. structure 100 (%) (μm) (μm) (%)(area %) (%) (mass ppm) (mass ppm) (MPa) (%) layer) thickness) (HV) (mm)ability Remarks 28 A ferrite + 1 1.5 15 95 3.8 1.2 15 30 400 42 360 395670 0.72 ○ Example cementite 29 A ferrite + 5 1.3 13 83 3.9 13.1 15 30400 32 360 400 670 0.73 ○ Comparative cementite + Example pearlite 30 Aferrite + 1 1.5 16 94 3.7 2.3 10 80 390 43 340 395 590 0.60 xComparative cementite Example 31 A ferrite + 25 0.4 10 92 3.5 4.5 15 30480 32 365 398 671 0.72 ○ Comparative cementite Example 32 B ferrite + 11.5 17 94 4.2 1.8 17 35 370 44 375 410 655 0.65 ○ Example cementite 33 Bferrite + 5 1.1 17 80 0.5 19.5 13 35 370 32 375 410 655 0.67 ○Comparative cementite + Example pearlite 34 B ferrite + 3 1.2 15 83 4.212.8 14 36 371 32 375 405 655 0.65 ○ Comparative cementite + Examplepearlite 35 C ferrite + 1 1.3 14 94 5.2 0.8 15 70 410 40 390 465 6800.63 ○ Example cementite 36 D ferrite + 1 2.0 17 94 4.5 1.5 10 50 385 41380 415 675 0.70 ○ Example cementite 37 E ferrite + 1 1.1 12 93 4.3 2.716 30 485 32 370 425 671 0.70 ○ Comparative cementite Example 38 Fferrite + 1 1.1 12 92 4.1 3.9 16 30 485 32 370 430 698 0.68 ○Comparative cementite Example 39 G ferrite + 1 1.5 15 93 4.1 2.9 14 30370 45 370 430 690 0.65 ○ Example cementite 40 H ferrite + 1 1.5 14 924.4 3.6 15 35 380 44 370 420 702 0.85 ○ Example cementite 41 I ferrite +1 1.1 10 91 4.1 4.9 13 45 490 32 370 420 710 0.80 ○ Comparativecementite Example 42 J ferrite + 1 1.3 14 93 4.3 2.7 16 35 365 45 340420 590 0.55 x Comparative cementite Example 43 K ferrite + 1 1.8 15 936.5 0.5 14 60 370 42 510 570 680 0.65 ○ Example cementite 44 L ferrite +1 1.5 13 93 6.2 0.8 12 200 370 43 380 480 580 0.65 x Comparativecementite Example 45 M ferrite + 1 1.3 15 93 6.7 0.3 0 70 390 43 470 530570 0.55 x Comparative cementite Example 46 N ferrite + 1 2.5 20 93 6.80.2 15 35 365 44 470 490 590 0.50 x Comparative cementite Example 47 Oferrite + 1 1.9 18 91 8.4 0.6 16 30 410 38 670 680 705 0.68 ○ Examplecementite 48 Q ferrite + 1 1.3 13 91 8.9 0.1 14 35 400 40 680 710 6750.70 ○ Example cementite 49 R ferrite + 1 1.4 13 91 8.2 0.8 15 40 400 39685 705 670 0.67 ○ Example cementite 50 S ferrite + 1 1.4 15 94 3.0 3.014 35 360 45 310 380 700 0.60 x Comparative cementite Example 51 Tferrite + 25 1.5 12 50 10.5 39.5 14 35 490 32 680 720 700 0.68 ○Comparative cementite + Example pearlite

TABLE 4 Hardness at 0.1 mm deep Hardness from surface Effective caseafter 70° C. layer after depth after oil cooling carburizing andcarburizing and C content (HV) quenching (HV) quenching (mm) C < 0.20%≥340 ≥600 ≥0.60 0.20% ≤ C < 0.30% ≥350 ≥600 ≥0.60 0.30% ≤ C < 0.35% ≥400≥600 ≥0.60 0.35% ≤ C < 0.40% ≥490 ≥600 ≥0.60 0.40% ≤ C < 0.45% ≥580 ≥600≥0.60 0.45% ≤ C < 0.50% ≥670 ≥600 ≥0.60 0.50% ≤ C ≥700 ≥650 ≥0.60

The results in Table 2-2 and Table 3-2 show that the high-carbonhot-rolled steel sheets of Examples each have a microstructure includingferrite and cementite, the proportion of the number of cementite grainshaving an equivalent circle diameter of 0.1 μm or less to the totalnumber of cementite grains being 20% or less, the average cementitegrain size being 2.5 μm or less, the cementite accounting for 3.5% ormore and 10.0% or less of the entire microstructure, and have both highcold workability and high hardenability. In addition, the high-carbonhot-rolled steel sheets of Examples were provided with excellentmechanical properties, i.e., a tensile strength of 480 MPa or less and atotal elongation (El) of 33% or more.

In contrast, in Comparative Examples outside the scope of the presentinvention, any one or more of the chemical composition, themicrostructure, the amount of solute B, and the amount of N in AlN donot satisfy the scope of the present invention, and as a result, thetarget performance described above cannot be satisfied in any one ormore of the cold workability and the hardenability. In some ComparativeExamples, the target properties were not satisfied in one or more of thetensile strength (TS) and the total elongation (El). For example, inTable 2-2 and Table 3-2, Steel S has a C content lower than the rangeaccording to aspects of the present invention and thus does not satisfythe immersion-quench hardenability. Steel T has a C content higher thanthe range according to aspects of the present invention and thus doesnot satisfy the TS and total elongation of the steel sheet.

1. A high-carbon hot-rolled steel sheet having a chemical compositioncomprising, by mass %, C: 0.20% or more and 0.50% or less, Si: 0.8% orless, Mn: 0.10% or more and 0.80% or less, P: 0.03% or less, S: 0.010%or less, sol. Al: 0.10% or less, N: 0.01% or less, Cr: 1.0% or less, B:0.0005% or more and 0.005% or less, and one or two selected from Sb andSn in an amount of 0.002% or more and 0.1% or less in total, with thebalance being Fe and unavoidable impurities, the steel sheet having amicrostructure including ferrite, cementite, and pearlite that accountsfor 6.5% or less of the entire microstructure by area fraction, whereinregarding the cementite, a proportion of a number of cementite grainshaving an equivalent circle diameter of 0.1 μm or less to a total numberof cementite grains is 20% or less, an average cementite grain size is2.5 μm or less, and the cementite accounts for 3.5% or more and 10.0% orless of the entire microstructure by area fraction, an averageconcentration of solute B in a region extending from a surface layer toa depth of 100 μm is 10 mass ppm or more, and an average concentrationof N present as AlN in the region extending from the surface layer tothe depth of 100 μm is 70 mass ppm or less.
 2. The high-carbonhot-rolled steel sheet according to claim 1, having a tensile strengthof 480 MPa or less and a total elongation of 33% or more.
 3. Thehigh-carbon hot-rolled steel sheet according to claim 1, wherein theferrite has an average grain size of 4 to 25 μm.
 4. The high-carbonhot-rolled steel sheet according to claim 2, wherein the ferrite has anaverage grain size of 4 to 25 μm.
 5. The high-carbon hot-rolled steelsheet according to claim 1, wherein the chemical composition furthercomprises, by mass %, one or two groups selected from Group A and GroupB: Group A: Ti: 0.06% or less, and Group B: one or two or more selectedfrom Nb, Mo, Ta, Ni, Cu, V, and W each in an amount of 0.0005% or moreand 0.1% or less.
 6. The high-carbon hot-rolled steel sheet according toclaim 2, wherein the chemical composition further comprises, by mass %,one or two groups selected from Group A and Group B: Group A: Ti: 0.06%or less, and Group B: one or two or more selected from Nb, Mo, Ta, Ni,Cu, V, and W each in an amount of 0.0005% or more and 0.1% or less. 7.The high-carbon hot-rolled steel sheet according to claim 3, wherein thechemical composition further comprises, by mass %, one or two groupsselected from Group A and Group B: Group A: Ti: 0.06% or less, and GroupB: one or two or more selected from Nb, Mo, Ta, Ni, Cu, V, and W each inan amount of 0.0005% or more and 0.1% or less.
 8. The high-carbonhot-rolled steel sheet according to claim 4, wherein the chemicalcomposition further comprises, by mass %, one or two groups selectedfrom Group A and Group B: Group A: Ti: 0.06% or less, and Group B: oneor two or more selected from Nb, Mo, Ta, Ni, Cu, V, and W each in anamount of 0.0005% or more and 0.1% or less.
 9. A method formanufacturing the high-carbon hot-rolled steel sheet according to claim1, the method comprising: subjecting a steel having the chemicalcomposition to hot rough rolling, then performing finish rolling at afinishing temperature equal to or higher than an Ar₃ transformationtemperature, and then performing cooling to 650° C. to 750° C. at anaverage cooling rate of 20° C./sec to 100° C./sec; performing coiling ata coiling temperature of 500° C. to 700° C. to obtain a hot-rolled steelsheet; and then heating the hot-rolled steel sheet in a temperaturerange from 450° C. to 600° C. at an average heating rate of 15° C./h ormore and performing annealing that involves holding at an annealingtemperature lower than an Ac₁ transformation temperature for 1.0 h ormore.
 10. The method for manufacturing the high-carbon hot-rolled steelsheet according to claim 9, having a tensile strength of 480 MPa or lessand a total elongation of 33% or more.
 11. The method for manufacturingthe high-carbon hot-rolled steel sheet according to claim 9, wherein theferrite has an average grain size of 4 to 25 μm.
 12. The method formanufacturing the high-carbon hot-rolled steel sheet according to claim9, wherein the chemical composition further comprises, by mass %, one ortwo groups selected from Group A and Group B: Group A: Ti: 0.06% orless, and Group B: one or two or more selected from Nb, Mo, Ta, Ni, Cu,V, and W each in an amount of 0.0005% or more and 0.1% or less.
 13. Amethod for manufacturing the high-carbon hot-rolled steel sheetaccording to claim 1, the method comprising: subjecting a steel havingthe chemical composition to hot rough rolling, then performing finishrolling at a finishing temperature equal to or higher than an Ar₃transformation temperature, and then performing cooling to 650° C. to750° C. at an average cooling rate of 20° C./sec to 100° C./sec;performing coiling at a coiling temperature of 500° C. to 700° C. toobtain a hot-rolled steel sheet; and then heating the hot-rolled steelsheet in a temperature range from 450° C. to 600° C. at an averageheating rate of 15° C./h or more and performing annealing that involvesholding at a temperature equal to or higher than an Ac₁ transformationtemperature and equal to or lower than an Ac₃ transformation temperaturefor 0.5 h or more, followed by cooling to a temperature lower than anAr₁ transformation temperature at an average cooling rate of 1° C./h to20° C./h, and holding at a temperature lower than the Ar₁ transformationtemperature for 20 h or more.
 14. The method for manufacturing thehigh-carbon hot-rolled steel sheet according to claim 13, having atensile strength of 480 MPa or less and a total elongation of 33% ormore.
 15. The method for manufacturing the high-carbon hot-rolled steelsheet according to claim 13, wherein the ferrite has an average grainsize of 4 to 25 μm.
 16. The method for manufacturing the high-carbonhot-rolled steel sheet according to claim 13, wherein the chemicalcomposition further comprises, by mass %, one or two groups selectedfrom Group A and Group B: Group A: Ti: 0.06% or less, and Group B: oneor two or more selected from Nb, Mo, Ta, Ni, Cu, V, and W each in anamount of 0.0005% or more and 0.1% or less.